Photovoltaic (PV) devices, such as solar cells, are important for the conversion of solar and thermal energy into electricity. Conventional PV devices are based on semiconductor p-n junctions. For a single-junction cell with one band-gap (at a concentration of one sun), the maximum theoretical conversion efficiency is about 30%, and a current state-of-the-art efficiency of about 28% at higher concentrations (i.e. >500 suns). To achieve high conversion efficiency, multiple junction cells with different band-gap materials can be used. For example, triple-junction solar cells (containing materials with three different energy gaps) have a theoretical conversion efficiency of 56% at 1,000 suns, and a current state-of-the-art efficiency of about 32% at 1 sun and approximately 41% at 240 suns. One of the obstacles that affect practical PV device performance is a limitation in the availability of semiconductor materials with a range of bandgaps that adequately span the solar (or heat) spectrum and that can be effectively integrated within a single device or system, as well as current matching between multiple junctions.